People think git is immutable. It is not.
A local GIT repo can be changed (including its history) however you please. But once you have shared it with others you can't take that back. If you try to, then others will notice that the hashes mismatch and that their HEAD diffs uncleanly.
I know the term is infamous here, but GIT is essentially a blockchain. Each commit has a hash, which is based on the hashes of previous commits, forming a linked list (+ some DAG branching).
People conflate blockchains, distributed networks and cryptocurrencies.
So it relies on a human noticing and acting upon it. People not noticing backdoors being merged into the project is kinda the source of this problem.
You can't automate checks for malicious code.
But the bigger issue is that this is some theoretical system which is not present in most git repositories.
Wouldn't that catch most problems, and not generate too many false alarms?
Sure, in the project repo the branches are just simple text files that contain the hashes of the commits they point to.
So they are trivial to change in the project repo. But it is also trivial for the distro project to keep copies of the branch/tag info and check against those. I guess what you mainly care about are the previous release tags. They should never change after a release.
> Relying on people noticing (even with alarms) is just super fragile.
I'd say there's plenty of motivation now for the major distros to put infrastructure in place to automate this (keeping track of previous releases) and to actually keep looking at the alarms.
> You can find valid reasons why you'd want to do stuff which raises the alarms so that other people become deaf to them
I'm sure the attackers would try things like that.
But let's say you have an open source application/library that is part of Debian.
How common has it been in the past that the app/lib project had a bunch of tagged releases, and then wanted to rewrite the history so that the tagged releases now point to different commits? I assume it has been very uncommon, but maybe I'm wrong?
And even if that is the case, new infrastructure tools can keep local copies of the source code for previous releases, and check against that.
Repo checking is not trivial, perfect, or sufficient. But I'd say it's a necessary component in guarding against attacks.
The big challenge is still that there is so much code added/changed for each new release of apps/libs that it is very difficult to check against attacks. The obfuscated C contest has proven again and again how hard it is.